1. Field of the Invention
The present invention relates to the application of a coating medium by use of a spray device onto a moving surface, whereby in the direct application method the moving surface is the surface of a material web, specifically a paper or cardboard web, and in the indirect application method is the surface of a transfer element from which the coating medium is transferred to the material web.
2. Description of the Related Art
Devices for application of a coating medium by use of a spray device onto a moving surface are already known from EP 0 670 004 B1, WO 94/11116 and EP 0 435 904 B1. Generally, the objective of this type of spray applicator device is to apply the coating medium with as little excess as possible, and in the best case scenario even with no excess at all (1:1 application) to the moving surface. In the application methods described in the above referenced publications, the coating medium is atomized by use of air and is applied to the moving surface. After the coating medium has settled there, the carrier air is suction removed from the application area through channels surrounding the spray apparatus. Even with very low application base weights, very large volumes of air must be supplied to and suction removed from the moving surface in a given time period. It has been demonstrated in practice that, from a quality point of view, only an insufficiently satisfactory coating result can be achieved with the previously known coating methods. Despite subsequent smoothing of the applied layer, the layer still displays an uneven structure as well as a rough, sandpaper-like surface.
Faced with these challenges, the current invention provides a spray coating method with which coating layers having more uniform and smooth surfaces may be achieved.
In the area of the spray device, an atmosphere of a back moistening and/or moistening medium for atomized coating medium is maintained. The back moistening and/or moistening medium may be the vapor of a liquid which may also act as a carrier liquid for the coating medium which, prior to spraying, is liquid or viscous, for example steam, toluol vapor, etc. In the following explanation of the current invention, only water vapor, that is, steam, will be referred to as a back moistening and/or moistening medium for reasons of simplifying the discussion. However, this does not imply that the scope of protection for the method according to the invention is limited to the usage of water vapor.
The maintenance of a steam atmosphere in the area of the spray device is advantageous in several respects for achieving a high quality coating result. On the one hand, the atomized coating medium moves in an atmosphere from the spray device to the moving surface which prevents excessive escape of moisture from the coating medium droplets and/or particles, thereby preventing drying out at least the surface of these droplets and/or particles. Because of this, the droplets and/or particles maintain their liquidity and are more able to flow after coming into contact with the moving surface, which contributes to a smoother surface of the applied coating layer.
On the other hand, it is not necessary to suck the steam atmosphere from the application area. It is merely necessary to compensate for steam losses which are caused possibly by precipitation of the steam as moisture onto the moving surface or onto the coating layer, by supplying fresh steam. This will largely prevent turbulence in the application area, such as is caused in the state of the art equipment by air supply and by air suction removal and will have a positive effect upon the uniformity of the layer application.
The previously addressed precipitation of steam as moisture onto the coating layer has no negative effects, since the water in the coating layer is present anyway in the form of carrier liquids for the coating medium. On the contrary, the precipitated moisture improves the flowability characteristics of the coating medium in the applied layer, resulting in a smoother surface. The same also applies to all other carrier liquids commonly used in coating mediums, and their vapor, for example toluol and toluol vapor.
It must be emphasized that, in the previously referred to sense, the term xe2x80x9cdryingxe2x80x9d is to be understood as the escape of carrier liquid from the coating medium in the form of vapor, and the term xe2x80x9cback moisteningxe2x80x9d is to be understood as the supply of carrier liquid from the steam atmosphere into the coating medium droplets and/or particles. In the best case scenario, the nature of the steam atmosphere in the area of the spray device provides a balance between drying and back moistening for the atomized coating medium.
With the method of the current invention, powdery coating mediums can be converted. The steam in this instance condensates on the coating medium particles while they travel from the spray device to the moving surface, thereby moistening them, so that after making contact with the moving surface they are able to flow and produce a smooth coating layer.
In a further development of the invention, the spray device is located in a chamber into which the back moistening and/or moistening medium is fed by a supply device provided for this purpose.
The spray device may, for example, take the form of a single substance spray device. It is, however, also possible to utilize a two-substance spray device within the scope of the method according to the invention, which sprays the coating medium by use of steam. The steam used for atomization may, for example, alone serve in the formation of the steam atmosphere in the area of the spray device. It is, however, also possible to supply additional steam to the area of the spray device.
Depending upon the boundary conditions of each individual application, for example the consistency and structure of the coating medium that is to be sprayed, two-substance spray devices working according to the internal mixing principle, as well as the external mixing principle, may be utilized.
A rotary spray device may also be utilized whereby the coating medium is sprayed by a high speed rotary spray element by the effect of centrifugal forces acting upon it. The coating medium sprayed in this manner can be supplied to the moving surface, for example, by use of the steam. A ring slot-type steam outlet opening may, for example, be provided around the rotary spray element, from which the steam is emitted in the shape of a cone of a predetermined cone angle. A ring-shaped arrangement of several individual steam outlet openings may also be used.
In addition, or as an alternative, electrostatic forces may also be utilized in order to bring the atomized coating medium to the moving surface. For example, the rotary spray device may be connected to a predetermined electrical potential, so that the atomized coating medium droplets and/or particles leave the spray device electrically charged. The moving surface, or an element assigned to the moving surface, can be connected to ground potential so that an attractive force is exerted upon the charged droplets and/or particles.
Finally, a device may be provided prior to the area of application, viewed in direction of travel of the moving surface, which, by utilizing steam, removes an air boundary layer from the moving surface which carries it. This facilitates the coverage of the entire surface of the moving web/surface with coating medium, which in turn has a positive effect on the coverage quality of the applied coating layer.
If steam is supplied to the moving surface prior to the spray device when viewed in direction of travel, and is then suction removed after the spray device when viewed in direction of travel, a movement component may be imparted to the atomized coating medium which would favor its deposit on the moving surface in a way maintaining a smooth coating layer. The steam volumes required for this, compared to the previously discussed unfavorable air volumes, are considerably less. Thus, no deterioration in the uniformity of the coating result occurs.
Another aspect of the invention relates to a device for the application of a coating medium by use of a spray device onto a moving surface. Regarding the constructive details, design variations, and the advantages that are achievable with this device, reference can be made to the previous discussion regarding the coating method in accordance with the invention.